1. Field of the Invention
The present invention relates to a spark plug for use in an internal combustion engine.
2. Description of the Related Art
A spark plug used for ignition in an internal combustion engine, such as an automobile engine, is affected by a tendency of increasing combustion chamber temperature for the purpose of increasing engine output and improving fuel economy. In order to enhance ignition, in an increasing number of engines the discharge portion of a spark plug projects into a combustion chamber. In such applications, a discharge portion of the spark plug is exposed to high temperature, and thus spark-effected wear of the discharge portion tends to accelerate. In order to enhance spark-effected wear resistance of a discharge portion, many spark plugs have been proposed of the type in which a noble-metal chip containing a predominant amount of Pt, Ir, or a like element is welded to the tip of an electrode.
For example, Japanese Patent Application Laid-Open (kokai) No. 9-7733 discloses a spark plug in which a noble-metal chip contains a predominant amount of Ir and Rh so as to utilize the merit of Ir; i.e., high melting point, and simultaneously prevent oxidation volatilization of Ir at high temperature (about 900xc2x0 C. or higher), to thereby enhance wear resistance at higher temperatures.
According to recent tendencies, in addition to enhancement of wear resistance, enhancement of ignitability has also been required. In order to meet this demand, the diameter (chip diameter) of a discharge portion (noble-metal chip) is reduced, or the length of a discharge portion projecting from a center electrode is increased, whereby the discharge portion is slenderized to thereby reduce discharge voltage.
3. Problems to be Solved by the Invention
However, experiments conducted by the present inventors have revealed that, even when a noble-metal chip having excellent wear resistance as described in Japanese Patent Application Laid-Open (kokai) No. 9-7733 is used, the thus-slenderized discharge portion is still subject to wear. The present inventors conducted the following experiment. A 6-cylinder gasoline engine (having a piston displacement of 2000 cc) was equipped with a spark plug configured such that only the center electrode had a discharge portion implemented by a noble-metal chip containing a predominant amount of Ir and 20% by weight Rh. The gasoline engine was operated at a speed of 5000 rpm with the throttle opened completely and using unleaded gasoline as fuel. After twenty-hours of operation, the appearance of the discharge portion was observed. As shown in FIG. 6, the discharge portion exhibited unusual wear; specifically, the discharge portion is arcuately eroded at a circumferential side surface, which is not a discharge face. In FIG. 6, the discharge face is the top surface of the discharge portion. As understood from FIG. 6, such wear is unusual in terms of its form, and the cause for the wear cannot be explained by merely spark discharge and oxidation volatilization. Therefore, the problem cannot be solved by a conventional method designed to restrain merely spark-effected wear and simple oxidation wear of a discharge portion implemented by a noble-metal chip.
It is therefore an object of the present invention to provide a spark plug in which a discharge portion is formed of a noble-metal chip, the spark plug being capable of restraining unusual wear as markedly observed on a slenderized discharge portion in particular.
The above object of the present invention has been achieved by providing a spark plug comprising a center electrode and a ground electrode, which is disposed such that a side surface thereof faces an end face of the center electrode, and having a discharge portion having a discharge face formed, by welding of a noble-metal chip, on at least either the center electrode or the ground electrode at a position corresponding to a spark discharge gap, the spark plug being characterized in that:
D is 0.3-0.8 mm; and
H is 0.4-2 mm, where D (mm) is a chip diameter representing an outside diameter of the noble-metal chip; and H (mm) is a discharge portion thickness representing a shortest distance between a periphery of the discharge face and a corresponding end edge of a weld zone where the noble-metal chip and the center electrode and/or the ground electrode are welded, the spark plug being further characterized in that:
the noble-metal chip contains a predominant amount of Ir and 0.5-8% by weight Ni.
The present inventors examined the unusually worn discharge portion shown in FIG. 6 and found that a deposit containing Ca and/or P was formed on the surface of the discharge portion. No unusual wear was observed on some discharge portions to which the deposit adheres. However, all of the discharge portions suffering unusual wear exhibited adhesion of the deposit induced by Ca and/or P. Therefore, such a deposit may be partially responsible for the above-mentioned unusual wear. As is apparent from FIG. 6, the unusual wear proceeds on a discharge portion only from a certain direction, implying that the unusual wear is partially caused by the existence of a certain fluid flow within an ignition atmosphere where the discharge portion ignites. For example, conceivably, the above-mentioned fluid may be a constant mixture flow (swirl flow) for uniformly diffusing fuel contained in the mixture. Also, the unusual wear may proceed from the above-mentioned two causes. In any case, the mechanism of such unusual wear is presumed to differ from that of wear arising as a result of melting or dispersion caused by spark discharge or that of wear arising as a result of simple oxidation volatilization on a discharge portion. Therefore, conventional methods have failed to completely restrain such unusual wear.
Focusing on the phenomenon that, as shown in FIG. 6, in the unusually worn portion the periphery of the discharge face is almost free from unusual wear, the present inventors analyzed the periphery of the discharge face for components and found Ni present in the periphery of the discharge face. The present inventors also analyzed the unusually worn portion (circumferential side surface) and found Ni absent there. That is, the Ni present in the periphery of the discharge face did not originate with fabrication of the noble-metal chip, but rather was introduced during use of the spark plug. Conceivably, repeated spark discharge causes Ni components to fly out from the ground electrode, which is formed of an Ni-based heat resistant alloy or a like metal; and the Ni components are subsequently injected into the periphery of the discharge face of the discharge portion. In any case, the present inventors acquired knowledge that, in an unusually worn discharge portion, a zone (the periphery of a discharge face) that is not susceptible to unusual wear contains Ni.
The present inventors found that the unusual wear is likely to occur particularly on a discharge portion slenderized, for enhancement of ignition, such that the outside diameter thereof (noble-metal chip diameter) is reduced, and the length of projection thereof from the center electrode is increased. Specifically, a spark plug having a slenderized discharge portion is a spark plug whose discharge portion assumes the following chip diameter D (mm) and thickness H (mm): D: 0.3-0.8 mm, H: 0.4-2 mm; particularly, D: 0.3-0.6 mm, H: 0.5-2 mm. A chip diameter D in excess of 0.8 mm or a discharge portion thickness H less than 0.4 mm departs from the concept of slenderizing the discharge portion. Such dimensions (D greater than 0.8 mm, H less than 0.4 mm) are unlikely to result in the aforementioned unusual wear, and are therefore outside the scope of the present invention. A discharge portion thickness H of not less than 0.5 mm is particularly preferred for yielding the effect of slenderization. A chip diameter D less than 0.3 mm fails to maintain sufficient durability against ordinary wear induced by spark discharge, oxidation volatilization, etc. The present invention is not intended to solve such a problem. When the discharge portion thickness H exceeds 2 mm, the discharge portion tends to accumulate heat excessively and thus wears abruptly. Such wear cannot be restrained even through application of the present invention. Therefore, the chip diameter D is specified as not less than 0.3 mm, and the discharge portion thickness H is specified as not greater than 2 mm.
The above-described slenderized discharge portion is markedly susceptible to unusual wear, conceivably for the reason that, as a result of slenderization, the discharge portion becomes likely to assume a high temperature. Presumably, an increase in temperature of the discharge portion activates the aforementioned presumed causes (deposit and a flow of mixture caused by fuel) for unusual wear. Therefore, the present invention is favorably applicable not only to a spark plug whose discharge portion is slenderized, but also to a spark plug configured such that an increase in temperature of the discharge portion is unavoidable due to poor heat release from the discharge portion. The present inventors carried out extensive studies as described above and as a result found that, when a slenderized discharge portion; specifically, a discharge portion having a chip diameter D (mm) of 0.3-0.8 mm and a discharge portion thickness H (mm) of 0.4-2 mm, is formed of a noble-metal chip which contains a predominant amount of Ir (i.e., containing more than 50 weight percent of Ir) and 0.5-8% by weight Ni, the aforementioned unusual wear can be restrained, thereby completing the present invention. In this invention, xe2x80x98Ir base alloyxe2x80x99 means an alloy including more than 50 weight percent of Ir.
The Ni content of the noble-metal chip is 0.5-8% by weight. An Ni content less than 0.5% by weight fails to sufficiently exhibit the effect of restraining unusual wear. An Ni content in excess of 8% by weight is too high, resulting in impaired durability against ordinary wear caused by spark discharge. Accordingly, the noble-metal chip preferably contains 0.5-8% by weight Ni, more preferably 1-4% by weight Ni, for effectively preventing both unusual wear and wear caused by spark discharge and oxidation volatilization. The noble-metal chip is formed of an Ir base alloy, which contains a predominant amount of Ir. Because of good durability against spark discharge, an Ir base alloy can be favorably applied to the noble-metal chip.
The noble-metal chip may contain one or more elements selected from the group consisting of Pt, Pd, Rh, Ru and Re. In addition to restraint of unusual wear which is effected by the Ni component contained in an amount falling within the above-described range, the noble-metal chip formed of the above-mentioned Ir base alloy and containing such an element(s) can restrain oxidation volatilization of Ir at high temperature. Particularly preferably, Rh is contained in an amount of 0.5-40% by weight. An Rh content of less than 1% by weight is insufficient for yielding the effect of restraining oxidation volatilization, whereas an Rh content in excess of 40% by weight causes a drop in melting point of the noble-metal chip, with a resultant failure to effectively restrain wear caused by spark discharge. Accordingly, Rh contained in an amount falling within the above-described range can effectively restrain wear of the discharge portion other than the aforementioned unusual wear; i.e., wear caused by spark discharge, wear caused by oxidation volatilization of Ir, etc.
The noble-metal chip may contain at least either Ru or Re in an amount of 1-5% by weight. These elements are inferior to Ir but effective for enhancing durability against spark discharge, since they are higher in melting point than Rh. Since these elements are less likely than Ir to undergo oxidation volatilization at high temperature, addition of these elements is effective for enhancing high-temperature oxidation resistance. When their content is less than 1% by weight, the above-mentioned effects cannot be sufficiently obtained. When their content exceeds 5% by weight, spark-effected wear resistance and high-temperature oxidation resistance are impaired. Therefore, their preferred content falls within the above-mentioned range.
The noble-metal chip may contain an oxide (composite oxide) of an element(s) selected from the group consisting of Sr, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ti, Zr and Hf, to thereby more effectively restrain oxidation volatilization of Ir at high temperature. Preferably, the oxide(s) is contained in an amount of 0.5-3% by weight. An oxide content of less than 0.5% by weight fails to sufficiently yield the expected effect of preventing oxidation volatilization of an added metal element component(s). An oxide content in excess of 3% by weight may only impair heat resistance of the noble-metal chip. Preferably, at least either La2O3 or Y2O3 is contained as the above-mentioned oxide. Additionally, ThO2, ZrO2, or a like oxide can be favorably employed.
In order to achieve the above-described object, the present invention provides a spark plug comprising a center electrode and a ground electrode, which is disposed such that a side surface thereof faces an end face of the center electrode, and having a discharge portion having a discharge face formed, by welding of a noble-metal chip, on at least either the center electrode or the ground electrode at a position corresponding to a spark discharge gap, said spark plug being characterized in that: the noble-metal chip comprises an Ir base alloy containing 0.5-40% by weight of Rh and 1-4% by weight of Ni.
By using an Ir base alloy comprising 1 to 4 wt % of Ni and 0.5 to 40 wt % of Rh for the noble-metal chip, unusual wear such as scooping-out at the peripheral surface of the spark discharge portion can be effectively prevented. Further, wear of spark discharge portion caused by spark discharge and by oxidation volatilization of Ir other than by said unusual wear can be effectively prevented. As the result, a long-life spark plug can be provided. Additionally, 1 to 5 wt % of at least one of Ru and Re can be added to advantageously improve wear caused by spark discharge.